Crystal Keeper

Level 4 FIRE/Warrior Pendulum Effect Monster
1500 ATK / 1800 DEF

[ Pendulum Effect ]
Each turn, the first time an "Ultimate Crystal" monster(s) and/or "Crystal Beast" card(s) you control would be destroyed by card effect that turn, it is not destroyed.

[ Monster Effect ]

If a "Crystal Beast" monster you control battles an opponent's monster, during damage calculation (Quick Effect): You can Tribute this card from your hand or face-up field; your battling monster's ATK/DEF become double its original ATK/DEF during that damage calculation only, but it is destroyed at the end of this Damage Step.

Rulings – Yugipedia

Printings:

Flames of Destruction: FLOD-EN093 (Common) - 2018-05-03
2019 Gold Sarcophagus Tin Mega Pack: MP19-EN066 (Common) - 2019-08-29
Structure Deck: Legend of the Crystal Beasts: SDCB-EN010 (Common) - 2022-09-29

Prices:

Cardmarket: 0.04 €
TCGplayer: $ 0.07